The present invention relates to a write circuit for a disc drive. More particularly, the present invention relates to a headroom assistance circuit for an H-switch in a write circuit.
A magnetic disc, for use in a computer disc drive, is a flat circular platter with a magnetic surface on which data can be stored by selective polarization of portions of the magnetic surface. The presence or absence of polarity transitions between the polarized portions represents particular binary values. The magnetic surface of the disc may be magnetized permanently with north/south or south/north orientation by applying a magnetic field near the disc.
In a disc drive, the magnetic field for magnetizing the disc is created by a miniature horseshoe magnet which is suspended above the magnetically coated disc. The horseshoe magnet consists of a magnetic material surrounded by a conductive coil. The magnetic field is created by passing a current through the coil. The direction of the magnetic field is determined by the direction of the current through the coil. Thus, by changing the direction of the current through the coil, the host system can change the orientation of the magnetic field stored on the disc.
The density of the data stored on the disc is affected by how quickly the host system can change the direction of the current through the coil. Since the coil is basically an inductor, the rate of change of the current is approximated by the following equation: ##EQU1## Where V is the voltage across the coil, L is the inductance of the coil, and di/dt is the rate of change of current through the coil with respect to time. Thus, it can be seen that the rate of change of current through the coil is dependent on the voltage across the coil.
The current through the coil and the voltage across the coil are typically controlled by an H-switch connected to the magnetic write head. The H-switch consists of four transistors and various bias resistors. The transistors and resistors are balanced around the magnetic write head such that one transistor's emitter and another's collector are connected on each side of the magnetic write head. Depending upon the voltage applied to the H-switch, current flows from the emitter of one transistor through the magnetic write head and into the collector of another transistor on the opposite side of the magnetic write head. When the voltage applied to the H-switch is reversed, the current flows through a different pair of transistors in the opposite direction through the magnetic write head. The amount of voltage which can be placed across the magnetic write head is determined by the voltage supply available to the H-switch.
Present desktop computers provide a 5 volt supply which is sufficient to maintain a quick rate of change for the write head current. However, portable computers require write circuits which use lower voltages. When lower voltage supplies are used with present H-switch technology, the time needed for current changes (rise time) is greater, which degrades overwrite performance at higher data rates. Thus, the performance of computers is restricted by current H-switch technology.